a. Field of the Invention
The present invention relates to an electronically driven pressure boosting system that is used to boost the torque output of an internal combustion engine.
b. Related Art
One way to boost the torque and peak power provided by a reciprocating piston internal combustion engine, is to use a pressure boosting device to increase the mass airflow into the engine. The increased air supply then permits a greater amount of fuel to be combusted in each ignition event.
Examples of pressure boosting devices include turbochargers and superchargers. A turbocharger is driven entirely or partly by energy in the exhaust stream. This is an efficient use of otherwise mostly wasted energy, but such devices suffer from the limitation that the boost is not available or significant at low engine speeds (rpms). Often, a driver may demand high torque from an engine at low rpms, for example at the start of an overtaking manoeuvre. If the pressure boost device is driven only by exhaust gasses, then boosted torque will not be available at low rpms.
One way of dealing with the limitation is to provide an electrical motor connected to the turbocharger, which is energised when the turbo boost is insufficient. This type of electrically driven pressure boosting device is, however, expensive in terms of hardware cost. Another solution is to use a supercharger, that is, a compressor device that is driven by means other than an exhaust gas turbine, for example via a mechanical linkage to the engine, or by an electrical motor driven from the vehicle battery and/or battery charging system. Mechanical supercharger systems can however, be mechanically bulky and expensive. Electrically driven supercharger systems provide a lower cost and compact solution, but can require a significant amount of electrical energy when driven, for example, up to three times the current which can normally be supplied by a typical motor vehicle 12 volt battery. Motor vehicle alternators are typically specified to provide either all or most of the power requirement for the entire vehicle, the battery only being used to store sufficient electrical power to start the vehicle engine and occasionally deliver power when the accessory load exceeds the alternator output. Typical European vehicle alternators are specified to provide about 130 A of current, while an electrically powered supercharger can require in excess of 300 A. An alternator able to supply this much current is significantly more expensive, heavy and bulky than a conventional alternator.
Because the pressure boosting device cannot be 100% efficient, there will also be inevitable electrical and mechanical losses associated with the device, that can place significant mechanical and thermal stress on components within the device.
The expense of increasing the capacity of the vehicle battery and charging system, or the dealing with inherent thermal and mechanical limits of components within the pressure boosting device, to meet any level of driver demand can easily outweigh the benefits of using an electrically driven pressure charging device. Therefore it is important to drive such a device in an efficient manner, and within the limits of the vehicle electrical power supply, and thermal and mechanical limits of the device itself. At the same time, it is important to maximise the torque boost benefit perceived by the driver over as wide a range of driving conditions as possible. Because the level at which an electrically driven pressure boosting device is driven is essentially independent from the engine operating speed, it is therefore necessary to devise an appropriate control system for operating the pressure boosting device that takes account of the system""s limitations.
It is an object of the present invention to provide a convenient and economical electrical pressure boosting device and method for increasing the torque available from an internal combustion engine.
According to the invention, there is provided an air charge boosting system for an internal combustion engine, the system comprising an electrically driven pressure charging device, an electrical supply system for providing electrical power to drive the pressure charging device including a battery and an engine-driven battery recharger, a switch to connect and disconnect the battery and recharger and an engine control system for controlling the switch and the operation of the pressure charging device, wherein the engine control system is arranged to:
i) determine a capacity utilization of the electrical supply system; and
ii) control the switch to isolate at least partially the battery from the engine-driven battery recharger and drive the pressure charging device using the battery when said capacity utilization is above a first threshold.
Also according to the invention, there is provided a method of operating an air charge boosting system for an internal combustion engine, the system comprising an electrically driven pressure charging device, an electrical supply system for providing electrical power to drive the pressure charging device including a battery and an engine-driven battery recharger, a switch to connect and disconnect the battery and recharger and an engine control system for controlling the switch and the operation of the pressure charging device, wherein the method comprises the steps of using the engine control system to:
i) determine a capacity utilization of the electrical supply system; and
ii) control the switch to isolate at least partially the battery from the engine-driven battery recharger and drive the pressure charging device using the battery when said capacity utilization is above a first threshold.
Then, the battery recharger, preferably an alternator or similar device, may be used to supply additional electrical consumers other than the pressure charging device.
By driving the pressure boosting device mainly or entirely using battery supplied electrical power, other vehicle electrical consumer unit, such as lights, electrically heated windows, etc, can continue to be powered at their full rating by the alternator, thereby avoiding battery voltage drops which may occur owing to the potentially high electrical current demanded by the electrically driven pressure charging device.
The pressure charging device may be any type of device for boosting the air mass flow into the engine, whether partially or wholly driven by an electric power supply, but is most preferably an electrically driven supercharger.
Because the pressure charging device operation is restricted within allowable operating limits, this helps to avoid problems due to operation of the device beyond the allowable limits, thereby allowing more economical system design. This will in general also result in reduced electrical power consumption for the device. Both of these benefits contribute towards reducing system mass, volume and cost, and improving the electrical efficiency of the device.
In a preferred embodiment of the invention, the pressure charging device is an electronically driven supercharger.
This may have a number of rotary impeller vanes. The geometry of each vane is fixed, and the actual air charge delivered through the supercharger is determined by the rotation speed of the impeller vanes.
The method following step ii) may also comprise the step of using the engine control system to:
iii) control the switch to reconnect the battery to the engine-driven battery recharger when said capacity utilization is below a second threshold equal to or below the first threshold.
In order to avoid toggling of the switch when the capacity utilization is at either the first threshold or the second threshold, it is preferred if hysteresis is applied to one of both of these thresholds.
Following step ii) the engine control system may be used to monitor the battery voltage. If during step ii) the battery voltage then drops owing to operation of the pressure charging device, the engine control system can be used to control the output voltage of the battery recharger in response to the monitored battery voltage, so that when the battery and recharger are reconnected in step iii), any difference between the battery voltage and the voltage of the battery recharger owing to operation of the pressure recharging device is reduced.
Therefore, the battery can be reconnected to the battery recharger without a significant difference in battery and recharger voltages causing a sharp drop in the voltage used to power other electrical consumers, which could be noticed by a driver of the vehicle.
When the battery is reconnected to the battery recharger, the battery will start to be recharged, causing the output voltage of the battery recharger to rise.
The capacity utilization is determined by the engine control system according to current drawn from the battery recharger.
The capacity utilization may be determined by the engine control system according to the battery state-of-charge. In particular, the state-of-charge may be a predetermined minimum supply voltage for the battery.
In a preferred embodiment of the invention, the system includes a filter circuit between the battery and the battery recharger, in which in step ii) the battery and the battery recharger are partially isolated through the filter circuit. The filter circuit allows the battery to act as ballast for the alternator, helping to avoid sudden changes in the alternator voltage owing to sudden changes in electrical load on the alternator from electrical consumer units.
Also in a preferred embodiment of the invention, during step ii) the electrical control system monitors battery voltage against one or more electrical allowable operating limits. Then, if the battery voltage falls below an operating limit, the control system takes action to restrict the operation of the pressure charging device. In this way, the operation of the pressure charging device is controlled and, if necessary, restricted so as not to discharge unduly the battery.
It is particularly advantageous if the allowable operating limits include both a soft limit and a hard limit. The operation of the electrically driven charge boosting device then being such that the operation of the electrically driven charge boosting device is limited when the operation of the pressure charging device would exceed the soft limit, the limitation being such that the operation of the pressure charging device does not at a later time cause the hard limit to be exceeded. Such limitation may take several forms, but so that operation of the pressure boosting device is not suddenly or unexpectedly limited, it is preferred if the electrical drive to the boosting device is progressively limited as the corresponding operating parameter approaches the hard limit.
The air boosting system may comprise an electronically controlled throttle for controlling engine aspiration. The system may then: set the throttle position to regulate the aspiration of the engine; determine the actual air charge according to engine operating conditions; compare the desired air charge with the actual air charge; and drive the pressure charging device to boost aspiration of the engine in accordance with the comparison between the desired air charge and the actual air charge only when the throttle is wide open and when the torque demand cannot be met by natural aspiration alone.
The engine torque output at any given engine speed is then controlled by the throttle setting at throttle positions below wide open throttle. Once the throttle position reaches wide open throttle, the control system operates the pressure charging device according to the comparison between the desired air charge and actual air charge, for example in such a way as to reduce a difference between these air charges to zero in the case of steady state operation of the pressure charging device.
The pressure charging device operation is therefore limited to regions where engine torque demand cannot be met with natural aspiration alone. Furthermore, the efficiency of assisted aspiration is increased by avoiding restriction by a partly open throttle, and by matching the actual air charge to the desired air charge by the electrical control of the pressure charging device.